A) Field of the Invention
The present invention relates to a manufacture method for zinc oxide (ZnO) based compound semiconductor crystal and a ZnO based compound semiconductor substrate.
B) Description of the Related Art
ZnO based compound semiconductor such as ZnO is one of wide gap semiconductors and has a large exciton binding energy of about 60 meV. If a light emission element is formed by using ZnO based compound semiconductor as an active layer material, it should be possible to obtain a light emission element having an optical emission efficiency higher than that of a light emission element using gallium nitride based (GaN based) compound semiconductor as an active layer material.
ZnO based compound semiconductor material is therefore expected to be used as the material of an active layer of a blue color light emission element or ultraviolet light emission element.
In order to use ZnO based compound semiconductor material as the material of an active layer of a light emission element, it is necessary first to form a single crystal layer of this compound semiconductor.
A single crystal layer of ZnO compound semiconductor may be formed directly or via a template layer on an a-plane sapphire substrate or c-plane sapphire substrate by molecular beam epitaxy (MBE) or laser ablation vapor deposition, for example.
The crystalline structure of ZnO based compound semiconductor is a Wurtzite structure which is one of the hexagonal systems. Crystal growth on an a-plane sapphire substrate or c-plane sapphire substrate generally occurs along the −c-axis (oxygen (O) plane) direction. If a gallium (Ga) plane GaN film is used as a template layer, it is possible to grow ZnO based compound semiconductor along its +c-axis (zinc (Zn) plane) direction. It is also possible to grow the crystal along its +c-axis direction on a Zn plane ZnO substrate. In order to use ZnO based compound semiconductor as the material of an active layer of, e.g., a light emission element, it is desirable to form a single crystal layer with less crystalline defects. However, undesirable crystalline defects are often formed in a single crystal of ZnO based compound semiconductor.
Some of the present inventors have previously invented a crystal growth substrate, which is suitable for growing ZnO based compound semiconductor of a hexagonal system crystal structure. See, e.g., JP-A-2003-282602 and its counterpart U.S. Pat. No. 6,673,478, both of which are herein incorporated by reference in their entireties. This crystal growth substrate uses a sapphire substrate, but the sapphire substrate has no electric conductivity and an electrode cannot be formed on the substrate bottom.